Possible Solutions: Delete the Xcode project and recreate it from Unity.

Problem: Program received signal: “0”.

Possible Causes: If you get "Received memory warning... WARNING -> applicationDidReceiveMemoryWarning()" most likely you ran out of memory.

Possible Solutions (this is assuming you are using a device older than the iPhone 4):

Make sure scene only has an average of 7,000 tris, give or take a few thousand.

Make sure music is compressed, preferably 1 MB at most per file.

Keep texture dimensions low. 128x128 is sufficient for single characters or buildings. Use 32x32 or 16x16 for small objects. Greater resolutions are ok for texture atlases.

Use texture atlases.

Make sure textures are compressed to PVRTC format (in Import Settings), 2 bits if possible, or 4 bits if 2 looks too ugly. PVRTC compression will not take place if your texture’s dimensions are not in power of two. GUI images generally don’t need to be PVRTC compressed but mesh textures should be.

Remove mip-map generation (in Import Settings) if the texture is used for the GUI. In Unity 3, this is automatically done if you set your texture type to GUI.

Limit the usage of full-screen GUI backgrounds, they eat up a lot of memory.

Don't load everything all at once. For example, making an array of Texture2D and putting images there will cause Unity to load all of those all at once. Don't do this, unless you need all of them on-screen all at the same time, instead, use Resources.Load to load only the one you need at that time, then call Destroy on it when not needed anymore.

Keep particle count low. Try increasing the particle’s size to make up for the lowered particle count.

Review the scripts you have downloaded from the Internet. Most likely they were made for a desktop computer in mind, making use of lots of memory an iPhone device doesn’t have.

Possible Solution: Delete the app in the device (using Xcode Organizer) then install it again.

Problem: Receiving “SIGBUS” signal

Possible Causes: Most likely a null reference exception in the Unity scripts.

Possible Solution: Check your code in Unity for null reference exceptions. Use the backtrace to pinpoint where in your code the error happens.

Problem: Receiving “EXC_BAD_ACCESS” signal

Possible Causes: Most likely a null reference exception in the Unity scripts.

Possible Solution: Check your code in Unity for null reference exceptions. Unfortunately, Xcode does not produce a useful backtrace in this situation. You'll have to figure things out from the Unity editor.

Problem: Receiving “SIGSEGV” signal

Possible Cause 1: Most likely caused by out of memory errors.

Possible Solution: See notes on out of memory problem.

Possible Cause 2: Unity iPhone 1.7 has a problem with C# GetComponent and GetComponentsInChildren when used with generics.

Possible Solution: Use the non-generic format instead.

Problem: When creating a new build: Error building Player: FileNotFoundException: Could not find file ".../Temp/StagingArea/iPhone-Trampoline/Unity-iPhone.xcodeproj/User.pbxuser".
UnityEditor.HostView:OnGUI()

Possible Cause: Unknown

Possible Solution: Close Unity, delete the Temp folder in your project if it exists, then try making a build again.